Control Rod Drive Mechanism

ABSTRACT

A control rod drive mechanism includes an outer tube and a guide tube which is arranged in the outer tube and is held by the outer tube. Furthermore, the control rod drive mechanism includes a spool piece with an inner magnet coupling and a rotary shaft connected to the inner magnet coupling arranged internally. The lower end portion of the outer tube is supported by the spool piece in the spool piece. The outside diameter of the outer tube, through the length of the outer tube, is smaller than the inside diameter of the spool piece and the outer tube, in the radial direction of the outer tube, is not projected outside the inner surface of the spool piece. Therefore, the installation places of the O-rings which are a seal member are reduced and the time required for maintenance of the control rod drive mechanism can be shortened.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent applicationserial no. 2014-044931, filed on Mar. 7, 2014, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a control rod drive mechanism and moreparticularly relates to a control rod drive mechanism suitablyapplicable to a boiling water reactor.

2. Background Art

A boiling water reactor includes a reactor pressure vessel, and a coreshroud, an upper lattice plate, a core support plate, a separator, and asteam dryer installed in the reactor pressure vessel. A core in which aplurality of fuel assemblies are loaded is surrounded by the coreshroud. A core support plate is disposed below the core and is attachedto the core shroud. An upper lattice plate is disposed above the coreand is attached to the core shroud. The core is disposed between thecore support plate and the upper lattice plate. The separator isdisposed above the upper lattice plate and the steam dryer is disposedabove the separator.

A lower end of each fuel assembly loaded in the core is supported by aplurality of fuel supports installed on the core support plate. Theupper end of each fuel assembly is supported by the upper lattice plate.

A plurality of control rods withdrawing from and inserting into amongthe fuel assemblies in order to control the reactor power are disposedin the reactor pressure vessel. These control rods are respectivelyconnected to control rod drive mechanisms. Each control rod drivemechanism is stored separately in a control rod drive mechanism housinginstalled at the bottom of the reactor pressure vessel. Cooling waterserving as moderator and coolant is filled in the reactor pressurevessel.

An example of the control rod drive mechanism used in the boiling waterreactor is described in Japanese Patent Laid-Open No. 10(1998)-132977.The control rod drive mechanism includes an outer tube, a guide tube, apiston tube, a ball screw, a ball nut, a spool piece, an inner magnetcoupling, an outer magnet coupling, and a motor. A flange of the outertube is attached to a flange of the spool piece by a plurality of bolts,as described in FIG. 1 of Japanese Patent Laid-Open No. 10(1998)-132977.The guide tube is disposed in the outer tube and a lower end of theguide tube is supported by the outer tube. The piston tube is disposedin the guide tube. The ball screw is inserted in a hollow portion formedaround a central axis of the piston tube. The ball nut mates with theball screw and a lower end of the piston tube is put on a top face ofthe ball nut. An upper end portion of the piston tube is coupledremovably to a lower end portion of the control rod. A back sheet isprovided in the lower end portion of the ball screw. The back sheet isdisposed opposite to a top face of the back sheet receiving portioninstalled inside the outer tube in the lower end portion of the outertube. In the normal state of each control rod drive mechanism, a gap isformed between an undersurface of the back sheet and the top face of theback sheet receiving portion, and the undersurface of the back sheet isnot in contact with the top face of the back sheet receiving portion.

An inner magnet coupling to which a rotary shaft is attached is disposedinside the spool piece attached to the outer tube. The rotary shaft isconnected removably to the back sheet.

A cylindrical outer magnet coupling is disposed so as to surround alower end portion of the spool piece. The outer magnet coupling isconnected to the rotary shaft of the motor attached to the spool pieceby the support member. An outer magnet installed on the inner surface ofthe outer magnet coupling is opposite to an inner magnet installed onthe outer surface of an inner magnet bearing and a side wall of thespool piece is disposed between the inner magnet and the outer magnet.

The outer tube of the control rod drive mechanism is inserted into thecontrol rod drive mechanism housing installed on a bottom head of thereactor pressure vessel and the flange of the outer tube and the flangeof the spool piece which are connected by a bolt are attached to theflange of the control rod drive mechanism housing by other a pluralityof bolts (refer to FIG. 1 of Japanese Patent Laid-Open No.10(1998)-132977).

The control rod drive mechanism shown in FIG. 1 of Japanese PatentLaid-Open No. 10(1998)-132977 does not require a rotary shaft forconnecting a inner magnet coupling and an outer magnet coupling andspool piece forms a pressure boundary and prevents cooling water in thereactor pressure vessel from leaking to the outside. Particularly, anO-ring disposed between the flange of the control rod drive mechanismhousing and the flange of the spool piece and an O-ring arranged betweenthe flange of the outer tube and the flange of the spool piece preventthe cooling water from leaking to the outside.

The rotary power of the motor is transmitted to the inner magnetcoupling by the outer magnet coupling to rotate the ball screw. The ballnut moves up and down by the rotation of the ball screw and by doingthis, the control rod is inserted into the core or is withdrawn from thecore.

In the control rod drive mechanism shown in FIG. 1 of Japanese PatentLaid-Open No. 10(1998)-132977, as mentioned above, the flange of theouter tube and the flange of the spool piece which are combined by abolt are attached to the flange of the control rod drive mechanismhousing by other a plurality of bolts. On the other hand, in the controlrod drive mechanism shown in FIG. 1 of Japanese Patent Laid-Open No.10(1998)-132977, as shown in FIG. 5 of Japanese Patent Laid-Open No.10(1998)-132977, in the state that the flange of the outer tube isattached to the flange of the control rod drive mechanism housing by abolt, the flange of the spool piece may be attached to the flange of thecontrol rod drive mechanism housing by another bolt. In such astructure, an O-ring is disposed between the flange of the control roddrive mechanism housing and the flange of the outer tube andfurthermore, an O-ring is disposed between the outer surface of theouter tube and the inner surface of the spool piece.

Also in Japanese Patent Laid-Open No. 8(1996)-82690, a control rod drivemechanism having the same structure as that of the aforementionedcontrol rod drive mechanism described in Japanese Patent Laid-Open No.10(1998)-132977 is described.

Also in the control rod drive mechanism described in Japanese PatentLaid-Open No. 60(1985)-47987, the ball screw is rotated by the motor andthe ball nut mating with the ball screw moves up and down. As a result,the control rod is inserted into the core or is withdrawn from the core.In the control rod drive mechanism, the flange existing in the upper endportion of the cylindrical spool piece is attached to the flange of thecontrol rod drive mechanism housing and a cylinder body is attached tothe flange in the lower end portion of the spool piece. The lower end ofthe guide tube of the control rod drive mechanism is put on the top faceof the cylinder body and the guide tube is supported by the cylinderbody.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent Laid-Open No. 10 (1998)-132977

[Patent Literature 2] Japanese Patent Laid-Open No. 8(1996)-82690

[Patent Literature 3] Japanese Patent Laid-Open No. 60 (1985)-47987

SUMMARY OF THE INVENTION Technical Problem

In the conventional control rod drive mechanism (for example, thecontrol rod drive mechanism shown in FIG. 1 of Japanese Patent Laid-OpenNo. 10(1998)-132977) driven by motor which is applied to the boilingwater reactor, an O-ring made of rubber is installed between the flangeof the outer tube and the flange of the spool piece. For the O-ring, itis necessary to remove the spool piece from the outer tube and replaceit with a new O-ring at a predetermined frequency (for example, once per10 years) from the viewpoint of the life span of the raw material. Toremove the spool piece from the outer tube, it must remove the bolt forconnecting the flange of the control rod drive mechanism housing and theflange of the spool piece by applying crushing force to the O-ring madeof metal which is disposed between the flange of the control rod drivemechanism housing and the flange of the outer tube and holds theairtightness between the flanges.

As a result, when attaching the spool piece to the control rod drivemechanism housing by a bolt after replacing the O-ring disposed betweenthe flange of the outer tube and the flange of the spool piece, it isnecessary to confirm the gap between the control rod drive mechanismhousing and the flange of the outer tube to confirm the crushingcondition of the O-ring made of metal.

In the control rod drive mechanism shown in FIG. 1 of Japanese PatentLaid-Open No. 10(1998)-132977, as aforementioned, the flange of theouter tube and the flange of the spool piece which are connected witheach other, are attached to the flange of the control rod drivemechanism housing by other a plurality of bolts. Even when theconnection structure of such 3 flanges in the control rod drivemechanism shown in FIG. 1 of Japanese Patent Laid-Open No.10(1998)-132977 is exchanged for a connection structure of such 3flanges in the control rod drive mechanism shown in FIG. 5 of JapanesePatent Laid-Open No. 10(1998)-132977, that is, a connection structurewhere the flange of the spool piece is attached to the flange of thecontrol rod drive mechanism housing by another bolt in the state thatthe flange of the outer tube is attached to the flange of the controlrod drive mechanism housing by a bolt, and even, it is necessary toremove the bolt for connecting the flange of the control rod drivemechanism housing and the flange of the spool piece when exchanging theO-ring disposed between the outer surface of the outer tube and theinner surface of the spool piece. Therefore, even in this case, it isnecessary to confirm a gap between the control rod drive mechanismhousing and the flange of the outer tube to confirm the crushingcondition of the O-ring made of metal similarly to the aforementionedcase.

Furthermore, in the control rod drive mechanism driven by motor, theouter magnet coupling and the inner magnet coupling are used to transmitthe rotary power of the motor to the ball screw. The outer magnetcoupling and the inner magnet coupling basically do not requiremaintenance, so that only for the maintenance of the O-ring disposedbetween the flange of the outer tube and the flange of the spool piece,the spool piece needs to be removed from the control rod drive mechanismhousing and the outer tube. As a result, the time required for themaintenance operation of the O-ring becomes longer.

An object of the present invention is to provide a control rod drivemechanism capable of shortening the time required for maintenance.

Solution to Problem

A feature of the present invention for attaining the above object is acontrol rod drive mechanism comprising an outer tube, a guide tubedisposed in the outer tube, a spool piece in which a lower end portionof the outer tube is inserted, and supporting the outer tube and theguide tube, an inner magnet coupling disposed in the spool piece, a ballscrew which is rotated by the inner magnet coupling and is disposed inthe guide tube, a ball nut mating with the ball screw, a piston tubewhich is put on the ball nut and is connected to a control rod, a motordisposed below the spool piece and attached removably to the spoolpiece, and an outer magnet coupling which is disposed outside said spoolpiece, is connected to a rotary shaft of the motor, and is disposedopposite to the inner magnet coupling across the spool piece,

Wherein an outside diameter of the outer tube is smaller than an insidediameter of the spool piece through an entire length of the outer tubeand the outer tube has a structure that it is not projected outside aninner surface of the spool piece in a radial direction of the outertube.

According to the present invention, the outside diameter of the outertube is smaller than the inside diameter of the spool piece through anentire length of the outer tube and the outer tube has the structurethat it is not projected outside the inner surface of the spool piece inthe radial direction of the outer tube, so that it becomes unnecessaryto dispose O-rings which are a seal member between the outer surface ofthe outer tube and the inner surface of the spool piece, and further, itbecomes unnecessary to remove the spool piece from a control rod drivemechanism housing only for the maintenance of the O-rings. Thus, thetime required for maintenance of the control rod drive mechanism can beshortened.

Advantageous Effect of the Invention

According to the present invention, the time required for maintenance ofthe control rod drive mechanism can be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram showing a boiling water nuclear powergeneration plant.

FIG. 2 is a longitudinal cross sectional view showing a control roddrive mechanism according to 1 which is a preferred embodiment of thepresent invention applied to a boiling water nuclear power generationplant shown in FIG. 1.

FIG. 3 is an explanatory drawing showing an example of a mechanism ofpreventing rotation of a ball screw when the motor is removed in acontrol rod drive mechanism shown in FIG. 1.

FIG. 4 is a longitudinal cross sectional view showing a control roddrive mechanism according to embodiment 2 which is another preferredembodiment of the present invention.

FIG. 5 is a longitudinal cross sectional view showing a conventionalcontrol rod drive mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be explained below.

Embodiment 1

A control rod drive mechanism according to 1 which is a preferredembodiment of the present invention will be explained by referring toFIG. 2.

Before explanation of the control rod drive mechanism of the presentembodiment, a rough structure of the boiling water nuclear powergeneration plant to which the control rod drive mechanism of the presentembodiment is applied will be explained by referring to FIG. 1.

The boiling water nuclear power generation plant is divided into twosuch as a forced circulation boiling water nuclear power generationplant and a natural circulation nuclear power generation plant dependingon supply form of the cooling water to the core. In the forcedcirculation boiling water nuclear power generation plant, the coolingwater is supplied forcedly to the core by driving of a recirculationpump (or an internal pump) installed in the reactor. In the naturalcirculation nuclear power generation plant, in the reactor, therecirculation pump or the internal pump is not installed and the coolingwater is supplied to the core by the natural circulation. A chimney isinstalled in the reactor to increase the natural circulation force ofthe cooling water.

The control rod drive mechanism of the present embodiment can be appliedto the respective reactors of the forced circulation boiling waternuclear power generation plant and the natural circulation nuclear powergeneration plant. Here, the structure of the natural circulation nuclearpower generation plant to which the control rod drive mechanism of thepresent embodiment is applied will be explained by referring to FIG. 1.

A natural circulation nuclear power generation plant 1 includes areactor 2, a turbine 15, and a condenser 16. The reactor 2 has a reactorpressure vessel 3 and a core shroud 5, an upper lattice plate 53, a coresupport plate 54, a chimney 10, a separator 11, and a dryer 12 areinstalled in the reactor pressure vessel 3. At least, when the naturalcirculation nuclear power generation plant 1 is in operation, thereactor pressure vessel 3 is attached with a cover 4 on the upper endportion thereof to be sealed up. A core 6 disposed in the reactorpressure vessel 3 and in which a plurality of fuel assemblies 7 areloaded is surrounded by the core shroud 5. The core support plate 54 isdisposed below the core 6 and is attached to the core shroud 5. Theupper lattice plate 53 is disposed above the core 6 and is attached tothe core shroud 5. The core 6 is disposed between the core support plate54 and the upper lattice plate 53.

A lower end portion of each fuel assembly 7 loaded in the core 6 issupported by a plurality of fuel supports (not drawn) installed on thecore support plate 54. An upper end portion of each fuel assembly 7 issupported by the upper lattice plate 53.

The chimney 10 disposed above the upper lattice plate 53 includes acylindrical outer pipe 55 and a lattice member 56. The outer pipe 55 isattached to the upper end of the core shroud 5. The lattice member 56 isdisposed in the outer pipe 55 and is attached to the inner surface ofthe outer pipe 55. The lattice member 56 is structured so as to combinea plurality of partition plates in the lattice shape and is extendedfrom a lower end of the outer pipe 55 up to an upper end of the outerpipe 55. A plurality of coolant paths 57 with a square cross section areformed in the outer pipe 55 by the lattice member 56. The separator 11is disposed above the chimney 10 and the dryer 12 is disposed above theseparator 11.

A plurality of control rod guide pipes 8 are arranged below the core 6in the reactor pressure vessel 3. A plurality of control rod drivemechanism housings 9 pass through a bottom head of the reactor pressurevessel 3 and are attached to the reactor pressure vessel 3. A lower endof each control rod guide pipe 8 is connected to an upper end of eachcontrol rod drive mechanism housing 9.

A plurality of control rods 20 (refer to FIG. 2) inserted in andwithdrawn from among the fuel assemblies 7 loaded in the core 6 forcontrolling the reactor power are separately disposed in the respectivecontrol rod guide pipes 8. Each lower end portion of these control rods8 is coupled removably and individually to each upper end portion of thecontrol rod drive mechanisms 21. Each control rod drive mechanism 21 isseparately stored in the control rod drive mechanism housings 9. Coolingwater serving as moderator and coolant is filled in the reactor pressurevessel 3.

The turbine 15 is connected to a nozzle 13 formed in the reactorpressure vessel 3 by a main steam pipe 58. The condenser 16 is disposedbelow the turbine 15 and is connected to a nozzle 14 by a water feedpipe 17. A water feed pump 18 and a feed water heater 19 are installedon the water feed pipe 17.

The necessary number of control rods 20 are withdrawn from the core 6 bythe respective control rod drive mechanisms 21 and as the result, thereactor power of the natural circulation nuclear power generation plant1 rises up to 100% power which is the rated power. Thereafter, thenatural circulation nuclear power generation plant 1 is operatedpractically in 100% power until one operation cycle finishes. When thenatural circulation nuclear power generation plant 1 is in operation,the cooling water supplied to the core 6 flows in the respective fuelassemblies 7, and is heated by heat generated by nuclear fission of anuclear fuel material in a plurality of fuel rods included in each fuelassembly 7, and a part thereof becomes steam. As a result, a gas-liquidtwo-phase flow including cooling water and steam rises in each fuelassembly 7.

The gas-liquid two-phase flow rises in the respective coolant paths 57formed in the chimney 10 and flows into the separator 11. The separator11 separates the cooling water and steam included in the flowed-ingas-liquid two-phase flow. The separated steam flows into the dryer 12and furthermore moisture is removed by the dryer 12. The steam fromwhich the moisture is removed is discharged into the main steam pipe 58from the dryer 12. This steam (saturated steam) is introduced to theturbine 15 to rotate the turbine 15. The generator (not drawn) coupledwith the turbine 15 is also rotated to generate electric power.

The steam discharged from the turbine 15 is condensed to water by thecondenser 16. The water is supplied into the reactor pressure vessel 3through the water feed pipe 17 as feed water. The feed water ispressurized by the water feed pump 18, is heated by the feed waterheater 19, rises in temperature, and then is introduced into the reactorpressure vessel 3.

The feed water supplied into the reactor pressure vessel 3 by the waterfeed pipe 17 is mixed with the cooling water separated by the separator11 in the reactor pressure vessel 3. The cooling water mixed with thefeed water flows into a downcomer 59 with an annular cross section whichis formed between the inner surface of the reactor pressure vessel 3 andthe respective outer surfaces of the outer pipe 55 of the chimney 10 andthe core shroud 5. The cooling water moves down in the downcomer 59 andis supplied into each fuel assembly 7 in the core 6.

The supply of the cooling water into each fuel assembly 7 is performedby natural circulation of the cooling water. The natural circulationforce of the cooling water is generated due to the density differencebetween the gas-liquid two-phase flow at a low density rising in thecore 6 and the cooling water at a high density moving down in thedowncomer 59 and the natural circulation force of the cooling water ismore strengthened due to the chimney effect of the chimney 10.

The structure of the control rod drive mechanism 21 of the presentembodiment will be explained by referring to FIG. 2.

The control rod drive mechanism 21 includes a piston tube 23, a ballscrew 24, a ball nut 26, an outer tube 28, a guide tube 29, a spoolpiece 34, an inner magnet coupling 35, an outer magnet coupling 40, anda motor 39. The guide tube 29 is disposed in the outer tube 28 and alower end portion of the guide tube 29 is supported by the outer tube28. The piston tube 23 is disposed in the guide tube 29. An upper guide30 is attached to an upper end portion of the outer tube 28. The upperend portion of the guide tube 29 is made smaller in the outside diameterand a disc spring 32 is disposed outside the upper end portion with theoutside diameter of the guide tube 29 decreased. The disc spring 32 isdisposed between the guide tube 29 and the upper guide 30. A number of31 indicates a coil spring.

The ball screw 24 is inserted into a hollow portion formed on a centralaxis of the piston tube 23. The ball nut 26 mates with the ball screw 24and a lower end of the piston tube 23 is put on the top face of the ballnut 26. An upper end portion of the piston tube 23 is removably coupledto a lower end portion of the control rod 20.

The spool piece 34 is disposed below the control rod drive mechanismhousing 9 and a flange 38 of the spool piece 34 is removably attached toa flange 9A of the control rod drive mechanism housing 9 by a pluralityof bolts 42. An O-ring 45 made of metal which is a seal member isdisposed between the flange 38 of the spool piece 34 and the flange 9Aof the control rod drive mechanism housing 9 and the airtightnessbetween the flange 38 of the spool piece 34 and the flange 9A of thecontrol rod drive mechanism housing 9 is held.

The lower end of the outer tube 28 reaches in the spool piece 34 and issupported by a cylindrical support member 48 disposed in the spool piece34. The outside diameter of the outer tube 28 is smaller than the insidediameter of the spool piece 34 through an entire length of the outertube 28 and the outer tube 28 can be inserted into the spool piece 34.Therefore, the outer tube 28 is not projected outside the inner surfaceof the spool piece 34 in the radial direction of the outer tube 28. Theinside diameter of the spool piece 34 is practically equal to the insidediameter of the control rod drive mechanism housing 9.

The magnet couplings include the inner magnet coupling 35 and the outermagnet coupling 40. The inner magnet coupling 35 is disposed in thespool piece 34 and is attached to a rotary shaft 33. A separationdetection structure (a gear coupling 36) is attached to the lower endportion of the ball screw 24. The upper end of the rotary shaft 33 isconnected removably to the separation detection structure. A coil spring37 is disposed so as to surround the rotary shaft 33 between a springsupport 60 attached to the rotary shaft 33 and the separation detectionstructure. A bearing case 47 is disposed in the spool piece 34 betweenthe spring support 60 and the inner magnet coupling 35, and is attachedto the spool piece 34. The rotary shaft 33 is supported rotatably by aplurality of bearings installed in the bearing case 47.

The motor 39 is disposed below the spool piece 34 and is attachedremovably to the spool piece 34 by a support member 51. The outer magnetcoupling 40 is connected to the rotary shaft of the motor 39 and isdisposed between the spool piece 34 and the motor 39. A cylinder portionof the outer magnet coupling 40 is disposed so as to surround the spoolpiece 34 outside the spool piece 34. The outer magnet installed on theinner surface of the cylinder portion of the outer magnet coupling 40 isopposite to the inner magnet installed on the outer surface of the innermagnet coupling 35 across the side wall of the spool piece 34. The innersurface of the outer magnet coupling 40 is not in contact with the outersurface of the spool piece 34 and a gap is formed between the innersurface of the outer magnet coupling 40 and the outer surface of thespool piece 34. Furthermore, a holding brake 61 is disposed and attachedabove the motor 39 and is attached to the support member 51. The holdingbrake 61 prevents the rotation of the rotary shaft of the motor 39 whenthe rotation of the motor 39 is stopped and prevents the ball screw 24from rotating due to the own weight of the control rod 20, thuspreventing the control rod 20 from moving down.

When the control rod 20 is withdrawn from the core 6, the motor 39rotates in a direction capable of withdrawing the control rod 20 in thecontrol rod drive mechanism 21. The rotary power of the motor 39 istransmitted to the outer magnet coupling 40 to rotate the outer magnetcoupling 40. When the outer magnet coupling 40 is rotated, the innermagnet coupling 35 disposed in the spool piece 34 is also rotated in therotation direction of the outer magnet coupling 40. As a result, therotary shaft 33 is rotated and the ball screw 24 is also rotated in therotation direction of the outer magnet coupling 40. Since the rotationof the ball nut 26 is prevented by the guide tube 29, the ball nut 26mating with the ball screw 24 moves downward. The piston tube 23 put onthe ball nut 26 moves down and the control rod 20 moves down in thecontrol rod guide pipe 8. Therefore, the control rod 20 is withdrawnfrom the core 6 and the reactor power increases.

Further, the control rod 20 shown in FIG. 2 is withdrawn perfectly fromthe core 6 and exists in the lowest position.

Here, a conventional control rod drive mechanism 21C will be explainedby referring to FIG. 5. Only the portions different from the structureof the control rod drive mechanism 21 of the present embodiment amongthe structure of the control rod drive mechanism 21C will be explained.In the control rod drive mechanism 21C, a flange 43 in which a part ofthe outer tube 22 is formed in the lower end portion of the outer tube22. The flange 43 is attached to a flange 9A of the control rod drivemechanism housing 9 by a bolt 41. A flange 38A of a spool piece 34A isattached to the control rod drive mechanism housing 9 by a bolt 42passing through the flange 38A of the spool piece 34A and the flange 43of the outer tube 22. In the control rod drive mechanism 21C, aseparation detection structure installed on the control rod drivemechanism 21 includes a gear coupling (a back sheet) 25 and a gearcoupling 36. Furthermore, in the control rod drive mechanism 21C, theO-ring 45 made of metal is disposed between the flange 9A and the flange43 and O-rings 46 made of rubber are disposed between the inner surfaceof the spool piece 34A and the outer surface of the outer tube 22.

In such a control rod drive mechanism 21C, as aforementioned, even atthe time of exchange of the O-rings 46 made of rubber disposed betweenthe outer surface of the outer tube 22 and the inner surface of thespool piece 34A, the bolts 42 for connecting the flange 9A of thecontrol rod drive mechanism housing 9 and the flange 38A of the spoolpiece 34A need to be removed. Therefore, to confirm the crushingcondition of the O-ring 45 made of metal disposed between the flange 9Aand the flange 43, the gap between the flange 9A of the control roddrive mechanism housing 9 and the flange 43 of the outer tube 22 needsto be confirmed. In the control rod drive mechanism 21C, the spool piece34A needs to be removed from the flange 9A of the control rod drivemechanism housing 9 only for the maintenance of the O-rings 46 made ofrubber.

The control rod drive mechanism 21 of the present embodiment does notform the flange 43 in the outer tube 28 as the control rod drivemechanism 21C and connects the flange 9A of the control rod drivemechanism housing 9 and the flange 38 of the spool piece 34 by the bolts42. That is, in the control rod drive mechanism 21, the flange 43 doesnot exist between the flange 9A and the flange 38. In the control roddrive mechanism 21, the outer tube 28 is disposed only in the controlrod drive mechanism housing 9 and the spool piece 34. The outsidediameter of the outer tube 28 is smaller than the inside diameter of thespool piece 34 through the entire length of the outer tube 28 and theouter tube 28 is not projected outside the inner surface of the spoolpiece 34 in the radial direction of the outer tube 28 because the outertube 28 does not form the flange 43. As a result, in the control roddrive mechanism 21, it becomes unnecessary to dispose the O-rings 46made of rubber which are a seal member between the outer surface of theouter tube 22 and the inner surface of the spool piece 34 as in thecontrol rod drive mechanism 21C.

As a result, in the control rod drive mechanism 21, the maintenancefrequency can be reduced and the time required for maintenance can beshortened compared with the control rod drive mechanism 21C.

In the control rod drive mechanism 21, the flange 43 of the outer tube28 is unnecessary, so that the sealing places are decreased by one placecompared with the conventional control rod drive mechanism 21C and theplaces capable of leaking cooling water in the reactor pressure vessel 3is reduced by one place. This leads to the reliability improvement ofthe boiling water nuclear power generation plant.

In the control rod drive mechanism 21 of the present embodiment, thesealing places are decreased by one place compared with the control roddrive mechanism described in Japanese Patent Laid-Open No.60(1985)-47987. Namely, in the control rod drive mechanism described inJapanese Patent Laid-Open No. 60(1985)-47987, the seal members need tobe disposed between the flange in the upper end portion of the spoolpiece and the flange of the control rod drive mechanism housing andbetween the cylinder body attached to the flange in the lower endportion of the spool piece and the flange, respectively. In the controlrod drive mechanism 21, a seal member equivalent to the seal memberdisposed between the flange in the lower end portion of the spool piecein the control rod drive mechanism described in Japanese PatentLaid-Open No. 60(1985)-47987 and the cylinder body is unnecessary.Therefore, the time required for the maintenance of the control roddrive mechanism 21 of the present embodiment becomes shorter than thetime required for the maintenance of the control rod drive mechanismdescribed in Japanese Patent Laid-Open No. 60(1985)-47987.

Further, in the present embodiment, the connection of the flange 43 ofthe outer tube 22 and the flange 9A of the control rod drive mechanismhousing 9 by the bolt 41 in the conventional control rod drive mechanism21C becomes unnecessary, so that the operation of attaching and removingthe control rod drive mechanism 21 from the control rod drive mechanismhousing 9 for maintenance can be simplified and the time required forthis operation can be shortened.

Furthermore, in the control rod drive mechanism 21, the flange 43 is notformed in the outer tube 28 and the exchange of the O-rings 46 made ofrubber becomes unnecessary, so that the gear coupling 25 provided forremoval of the spool piece 34A and the metal seal by a back sheet 33formed in the outer tube 22 can be deleted. As a result, the coolingwater leakage potential of the control rod drive mechanism 21 can bereduced. Furthermore, the back sheet 33 can also be deleted, so that thenumber of parts of the control rod drive mechanism 21 can be reduced.

Further, the motor 39 and the holding brake 61 become a subject ofmaintenance, though the support member 51 is removed from the spoolpiece 34, thus the maintenance of the motor 39 and the holding brake 61can be performed easily. When performing the maintenance of the motor 39and the holding brake 61, there is no need to remove the spool piece 34from the control rod drive mechanism housing 9. However, when removingthe motor 39 and the holding brake 61 from the spool piece 34, thecontrol rod 20 is withdrawn perfectly from the core and the ball nut 26is positioned at the lowest position so that the ball screw 24 isrotated and the ball nut 26 does not move down due to the own weight ofthe control rod 20 connected to the piston tube 23.

When removing a motor portion 50 (refer to FIG. 3) including the motor39 and the holding brake 61 from the spool piece 34 for maintenance ofthe motor 39 and the holding brake 61 in the state that the ball nut 26is not positioned in the lowest position, a rotation prevention magnet49 is quickly attached to the outer surface of the spool piece 34 afterremoving the motor 39 and the holding brake 61 from the spool piece 34(refer to FIG. 3). The rotation prevention magnet 49 is a magnetbecoming magnetically paired with the inner magnet of the inner magnetcoupling 35. The rotation of the inner magnet coupling 35 due to the ownweight of the control rod 20 is prevented by the rotation preventionmagnet 49. Therefore, the descent of the control rod 20 is prevented.

In the nuclear power generation plant, even if one control rod 20 iswithdrawn perfectly from the core 6, the safety thereof is ensured.

In the control rod drive mechanism 21, the outer tube 28 is supported bythe support member 48 disposed in the spool piece 34, though the lowerend of the outer tube 28 may be supported directly by the spool piece34.

Embodiment 2

A control rod drive mechanism according to embodiment 2 which is anotherpreferred embodiment of the present invention will be explained byreferring to FIG. 4. A control rod drive mechanism 21B of the presentembodiment has a structure that a rotation prevention magnet 49A isadded to the control rod drive mechanism 21 according to embodiment 1.The rotation prevention magnet 49A is a magnet becoming magneticallypaired with the inner magnet of the inner magnet coupling 35 and isattached to the spool piece 34 in the position opposite to a part of theinner magnet coupling 35. Other structure of the control rod drivemechanism 21B is the same as to that of the control rod drive mechanism21 of embodiment 1.

In the control rod drive mechanism 21B, when the motor 39 and theholding brake 61 are removed from the spool piece 34 for maintenance,even when the ball nut 26 is not positioned in the lowest position, therotation of the inner magnet coupling 35 due to the own weight of thecontrol rod 20 is prevented by the rotation prevention magnet 49Abecause the rotation prevention magnet 49A is installed on the spoolpiece 34. Therefore, the descent of the control rod 20 is prevented.

The present embodiment can obtain each effect generated in embodiment 1.Furthermore, even when the motor 39 and the holding brake 61 are removedfrom the spool piece 34 for maintenance in the state that the ball nut26 is not positioned in the lowest position, in the present embodiment,the rotation of the ball screw 24 due to the own weight of the controlrod 20 can be prevented by the action of the rotation prevention magnet49A installed on the spool piece 34. Therefore, the descent of thecontrol rod 20 can be prevented.

The control rod drive mechanisms of embodiments 1 and 2 can be appliedalso to the forced circulation boiling water nuclear power generationplant.

REFERENCE SIGNS LIST

-   -   1: natural circulation nuclear power generation plant, 2:        reactor, 3: reactor pressure vessel, 6: core, 9: control rod        drive mechanism housing, 9A, 38: flange, 10: chimney, 20:        control rod, 21, 21B: control rod drive mechanism, 23: piston        tube, 24: ball screw, 26: ball nut, 28: outer tub, 29: guide        tube, 33: rotary shaft, 34: spool piece, 35: inner magnet        coupling, 39: motor, 40: outer magnet coupling, 45: O-ring, 49,        49A: rotation prevention magnet, 50: motor portion.

What is claimed is:
 1. A control rod drive mechanism including an outertube, a guide tube disposed in said outer tube, a spool piece in which alower end portion of said outer tube inserted, and supporting said outertube and said guide tube, an inner magnet coupling disposed in saidspool piece, a ball screw which is rotated by said inner magnet couplingand is disposed in said guide tube, a ball nut mating with said ballscrew, a piston tube which is put on said ball nut and is connected to acontrol rod, a motor disposed below said spool piece and attachedremovably to said spool piece, and an outer magnet coupling which isdisposed outside said spool piece, is connected to a rotary shaft ofsaid motor, and is disposed opposite to said inner magnet couplingacross said spool piece, Wherein an outside diameter of said outer tubeis smaller than an inside diameter of said spool piece through an entirelength of said outer tube and said outer tube has a structure that it isnot projected outside an inner surface of said spool piece in a radialdirection of said outer tube.
 2. The control rod drive mechanismaccording to claim 1, wherein a rotation prevention magnet which is amagnet becoming magnetically paired with said inner magnet coupling isinstalled in a position opposite to said inner magnet coupling of saidspool piece.